It could be that the low battery indication is false - it's difficult to measure accurately. You may find that it goes away.
The fob wakes up in a 125kHz field with a specific modulation. Each time this happens, a bit more battery energy is used. It could be that this is happening with a spurious source, but it's unlikely. Another common reason for batteries running out is pressing the transmit button when 'fiddling' with keys.
Variations like this and the number of uses mean that the battery life can be one to five or more years,